1. Field of the Invention
The present invention relates to a method and apparatus for adjusting the performance of an optical system in a projection optical apparatus and, more particularly, to a method and apparatus for highly precisely adjusting the imaging performance of a projection optical apparatus which is used to project a micropattern formed on a photomask or a reticle onto a semiconductor wafer.
2. Description of the Prior Art
A projection exposure apparatus using a projection lens can provide high alignment and matching precision and is therefore making a great contribution to the fabrication of VLSIs. A projection exposure apparatus of this type projects an image of a pattern on the photomask or the reticle onto the wafer coated with a photoresist at a predetermined factor by means of a projection lens. An important factor in the performance of the exposure apparatus is the matching precision. The most important factor influencing the matching precision is a magnification factor error of the projection optical system. The size of patterns used for the manufacture of VLSIs is becoming increasingly micronized these days, and demand for improvements in the matching precision is also strong. For this reason, it is more than ever important to keep the projection magnification factor at a predetermined value. With a projection optical apparatus available today, the magnification factor of the optical system is adjusted during installation of the apparatus so that the error in the magnification factor is kept negligible. However, in order to respond to the need for higher integration of VLSIs, any errors in the magnification factor of the projection optical system which arise from even the slightest changes in the barometric pressure or other environmental factors in a clean room having the apparatus installed must be corrected. A change in the magnification factor which arises due to a temperature change in the projection lens upon absorption of exposure energy must also be corrected. However, in general, when the magnification factor of the optical system is changed due to changes in environmental factors such as a change in barometric pressure or an increase in the temperature of the projection lens, the position of the imaging plane is also changed. A projection objective lens for which a high resolution is required has a large N.A. (numerical aperture) and a short focal depth. Therefore, even a slightest change in the position of the imaging plane must be corrected satisfactorily.
In order to change the projection magnification factor of the projection optical system, methods have been conventionally adopted wherein the distance between an object or an image surface and the projection lens is mechanically changed or the lens elements of the projection lens are moved along its optical axis. However, with the conventional method of mechanically moving the optical elements, when highly precise setting of the magnification factor and of imaging plane are required, the eccentricity (shift, tilt) of the movable portion prevents the optical axis from being kept constant. Therefore, the optical axis of the optical system including the object cannot be aligned on a single line, so that a magnification factor distribution which is asymmetrical with respect to the ideal optical axis is generated on the image surface. In order to set the magnification factor with a high precision such that an error within only 0.05 .mu.m or less is generated, the displacement of the optical elements must be controlled to be less than 1 .mu.m or at most several micrometers including the eccentricity (shift, tilt). However, such control involves extreme difficulties.